Since their inception, drive wheel dependent vehicle architectures have sought to improve the efficiency of power transfer from a set or sets of drive wheels to a given surface through traction enhancement. Traction (i.e., the ability of a given drive wheel to contact and transfer power to a given surface) directly impacts certain aspects of vehicular performance such as maneuverability of vehicle, fuel efficiency, accelerative torque distribution, deceleration, and/or special braking effectiveness. Most importantly perhaps, the greater the drive wheel traction of the vehicle, the more control an operator may exercise over the vehicle. Thus, maximizing traction is not only an important factor in maximizing vehicular performance, but it also provides a reliable and safer vehicle under all driving conditions.
Maintaining traction became problematic in earlier vehicular architectures when executing a turn. Specifically, traction was compromised when the front drive wheels of a vehicle, each spacially fixed apart relative to each other and coupled together by a common rotor, were required to travel unequal distances. For example, the relative paths of the frontal drive wheels of a two-wheel drive vehicle executing a turn can be conceptualized as two circles of different diameters. The non-drive wheels need not be considered in this example because they are allowed to rotate freely. During a right turn, the circular path of the left wheel is characterized as having a larger diameter than the circular path of the right drive wheel. Thus, the left drive wheel must travel a greater distance than would the right drive wheel. Unfortunately, earlier vehicular architectures required an equivalent rotational frequency or angular velocity to be present in both drive wheels at all times. A loss of traction, or wheel slippage, during a turn consequently occurred as both drive wheels, rotating at an equal frequency, were required to travel unequal distances. This problem was mitigated with the creation of the mechanical, gear-oriented differential, whereby a first drive wheel was permitted to rotate at a different rate with respect to a second drive wheel. This provided a reasonable solution to maintaining drive wheel traction during a turning action. Shortly thereafter, mechanical differentials became commonplace in vehicular design.
Unfortunately, the structure of the conventional mechanical differential operates such that the drive wheel with the lowest coefficient of traction limits the traction available to the other drive wheel. That is, if the first wheel is allowed to spin freely without providing a significant amount of traction, its counterpart drive wheel will remain virtually stationary. Thus, a worst-case traction scenario is created if only one of the two drive wheels looses traction in a two-wheel drive vehicle.
In more recent years, it became clear that it would be desirable to provide a second set of drive wheels. This significantly increased total traction. If the front drive wheels are capable of being engaged or disengaged by the operator, the vehicle is of the four-wheel drive type. If all drive wheels remain powered at all times, the vehicle is of the all-wheel drive type. For the aforementioned reasons, it is common practice that all-wheel drive systems utilize a first mechanical differential between the front drive wheels and a second mechanical differential between the rear drive wheels.
Regrettably, all-wheel drive and front-wheel drive systems exhibit several undesirable characteristics unique to their mechanical design. Amongst these are the inability to produce a regenerative breaking action and an inability to be disconnected during an Automatic Braking System (herein after ABS) breaking event. Furthermore, mechanical all-wheel drive systems require a drive-shaft tunnel extending the length of longitude in the wheel base, adding significant mass to the vehicle, decreasing fuel efficiency, and increasing the probability of mechanical failure. Ultimately, four-wheel and all-wheel driven vehicles, while improving vehicular performance through traction enhancement, are still limited by the conventional mechanical differential they employ.
A worst-case traction scenario arises on a four-wheel or all-wheel driven vehicle when one or both of the front wheels maintains tractions while one or both of the rear drive wheels looses traction. Unfortunately, such situations occur frequently without any means of counteracting or compensating for lost traction. Such a compromise in traction may occur anytime the vehicle contacts a surface having a split coefficient of friction, i.e., one drive wheel contacts a surface having a first coefficient of friction and a second drive wheel contacts a surface having a second, different coefficient of friction. A split-coefficient surface may be produced by natural elements such as snow, sleet, ice, rain, and or patches of dirt or gravel, or by synthetic elements such as through spillage of viscous fluids, such as oil. When encountering such a split coefficient surface, a loss of traction is generally experienced by the vehicle, thus compromising performance and control.
One known way to counter this problem involves coupling an individual motor to each drive wheel, thereby permitting each drive wheel to operate independently of the others, altering its angular velocity as conditions necessitate. Thus, a vehicle experiencing a loss of traction with a singular front-wheel and a singular back-wheel could effectively redistribute torque to the gripping wheels. Alternatively, a second innovation became known that acts similarly to a simple mechanical differential but with further able to sense a loss in traction and distribute torque accordingly. However, this torque sensing (“Torsen”) differential and the after mentioned independent traction motors both suffer from several significant disadvantages inherent in their mechanical nature. The complex machinery of the “Torsen” differential or the addition of weighty independent traction motors to a vehicle both degrade vehicular performance and operator control. Additionally, disadvantages of the mechanical differential systems include, but are by no means limited to, increased manufacturing costs, substantial maintenance costs, decreased mechanical reliability (e.g., and increased probability of mechanical failure and vehicular break-down), and due to the substantial weight increase, loss of fuel efficiency and degradation of vehicular maneuverability.
In view of the foregoing, it should be appreciated it would be desirable to provide an improved apparatus capable of creating a differential angular velocity between two given drive wheels for use in a vehicle. In an all-wheel vehicle system, it would be of additional benefit if torque could be created rather than distributed over both sets of drive wheels. Finally, it would be desirable to provide an automotive electrical differential system capable of being disconnected during an ABS braking event and/or producing a regenerative braking action. Additional desirable features will become apparent to one skilled in the art from the foregoing background of the invention and the following detailed description of a preferred exemplary embodiment and the appended claims.